The role of carbohydrate in the action of thyrotropin (TSH) and other glycoprotein hormones has not been clearly elucidated. Using newly developed methods of enzymatic deglycosylation we have observed significantly decreased bioactivity of deglycosylated TSH, both in terms of potency and V(max). However, the effects vary in different assays, and are less than those previously reported for chemical deglycosylation. The mechanism for these effects appears to be related to a change in hormone conformation rather than a direct effect of carbohydrate with the receptor. Certain patients with thyrotropic tumors show serum TSH with enhanced bioactivity, probably resulting from altered glycosylation.